1. Field of the Invention
In general, the invention relates to vacuum contactors employing interrupters and in particular to DC electromagnets utilized to close the vacuum contactor.
2. Description of the Prior Art
There are many designs of vacuum interrupters in existence. U.S. Pat. No. 4,002,867, issued Jan. 11, 1977 entitled "Vacuum Type Circuit Interrupters With a Condensing Shield at a Fixed Potential Relative to the Contact" is a representative example of such vacuum interrupters. An operating mechanism combined with one, two or three vacuum interrupters constitutes a vacuum contactor. In contradistinction to circuit breakers which are considered as principal protective devices during fault conditions in an electrical circuit and are designed for 20,000 to 50,000 operations, the vacuum contactor is used to start and stop various electric loads in response to signals generated by control devices such as push button switches, limit switches, and programmable controllers with the vacuum contactor being designed to have a lifetime of 2 to 3 million operations.
The main difference between vacuum contactors and conventional air break contactors is that the vacuum interrupters of the vacuum contactor break or interrupt the electric current inside a vacuum chamber instead of inside an air arc box. The vacuum chamber for the vacuum interrupter consists of a unit assembly of a sealed evacuated enclosure surrounding a fixed or stationary electrical contact and a moveable electrical contact. A portion of the moveable contact extends through a gas-tight metallic bellows which allows for the essentially linear motion of the moveable contact with respect to the stationary contact. The bellows is attached to the evacuated chamber by means of an end seal. Another end seal is provided for attaching the stationary contact to the enclosure. The ceramic sleeve or cylinder is provided to separate and electrically update the two contacts. The end seals are attached to the ends of the ceramic sleeve forming the evacuated chamber of the vacuum interrupter.
Because vacuum interrupters are normally closed by atmospheric pressure and an auxiliary contact spring, means must be provided to force the contacts into the open position which is the normal state for a deenergized contactor. The actual contact force holding the moveable and stationary contacts together inside each vacuum interrupter is the sum of the atmospheric force (atmospheric pressure times the mean area of the bellows) plus the force provided by the auxiliary contact spring and the mechanical spring force exerted by the bellows. This auxiliary contact spring force increases the total force sufficiently to sustain closure of the contacts during high short circuit currents that tend to blow the contacts apart. In the deenergized condition, there is no electrical energy available to provide the force necessary to separate the contacts. Instead, one or more mechanical springs provide this contact opening force. In practice this spring, called the kickout spring, exerts sufficient force to maintain the contacts in the open position in a deenergized contactor. To close the contacts of the vacuum interrupter on command, an electromagnet is provided that when energized, will pull the operating mechanism closed, overcoming the force of the kickout spring and closing the contacts of the vacuum interrupter.
It has generally been known that it should be possible to make DC excited electromagnets smaller that AC excited magnets for a given tractive pull. Since electromagnets are used by the thousands in industry, their optimization is a matter of importance. However, the potential capability of DC electromagnets has not been attained in the past; in fact, their capability has been less than AC versions of the same physical size.
An evaluation of electromagnets compares the magnetic force (pull) per watt of electrical energy consumed, all other parameters being equal. The electrical energy is consumed in the production of magnetic flux across the airgap of the electromagnet, which flux requires excitation of the magnet by a magnetomotive force expressed in ampere turns, i.e., current through the magnet coil times the number of turns in the magnet coil.
In an alternating current (AC) magnet the flux across the airgap alternates, producing a net magnetic force whose value cyclically varies at twice line frequency, producing chattering and not much effective force. Therefore, short circuited loops called shading coils are embedded in a portion of each magnet face to force the airgap flux into two time-phased components that do not go to zero simultaneously and therefore create a quiet, almost steady net pull. In such Ac magnets there are four consumers of energy:
(1) The I.sup.2 R losses in the operating coil, where I is the coil current and R the coil DC resistance.
(2) The NI.sub.s.sup.2 R.sub.s shading losses, where I.sub.s is the current in the shading coils, R.sub.s is the shading coil resistance, and N is the number of shaders, usually two.
(3) Eddy current core losses in the magnet iron due to induced currents in the laminations, rivets, and other magnetic loops.
(4) Hysteresis core losses in the magnet iron due to cyclical reversal of the flux. This is a function of the basic raw material, its thickness, and its heat treatment.
The above is not intended to be an exact description of a shaded AC magnet, but instead is a statement of the various types of losses involved in an AC magnet. As will be described hereinafter, only I.sup.2 R losses, Item 1, occur in a DC magnet.
With regard to item 2, shading losses, it is not practical to reduce them to zero, since the time displacement between the two flux components would also reduce to zero, and the magnet would become both noisy and weak. A trade-off must be made in shaded area of magnet face, shading coil impedance, and shading watts for acceptable magnet noise and pull.
There is a compensating advantage for all these losses. When the AC magnet is at open gap, its coil impedance is low, the operating coil current is high, and the ampere turn excitation is high at a time when high excitation is desirable for pick-up. The operating coil current for an AC magnet is closed, the impedance increases, the exciting current decreases, and watts loss decreases acceptably.
In a direct current (DC) magnet the flux across the airgap does not alternate, no shading coils are required, and the magnet need not be laminated to reduce core losses (which do not exist on DC). The current in the operating coil is limited only by the DC resistance of the coil and is therefore independent of the magnet gap. The core of the DC magnet can be laminated to reduce eddy currents. However, lack of eddy currents reduces magnetic damping of the closing mechanism to a minimum inducing mechanical transients, i.e., vibration, upon closing of the vacuum contactor. This causes chattering of the closing mechanism that adversely effects the performance of the vacuum contactor.
Stated slightly differently, the ampere turns available for pick-up on open gap (i.e., the vacuum contactor is open) on a DC magnet is the same as the ampere turns on closed gap (i.e., the vacuum contactor is closed). Because more ampere turns are required to establish the flux across an open gap than a closed gap, the designer must choose between making an inefficient DC magnet whose closed gap pull is more than required (in order to obtain the open gap pull needed), or increasing the closed gap resistance of the coil circuit to mimic the automatic impedance change of an AC magnet. This has been done in the past by inserting resistance into the coil current by means of an auxiliary contact as the magnet closed. In more recent times an improved arrangement using a two winding coil and integral rectifier has been developed. For an example refer to U.S. Pat. No. 4,223,289, entitled "AC-DC Magnet Coil Assembly For Low Dropout AC Contactors," issued Sept. 16, 1980 and assigned to the assignee of the present invention.
The cost of energy to operate a magnet is generally not a key reason for concern with the watts loss. Rather, the concern is that the watts loss is dissipated as heat, with the maximum temperature limited by the insulation of the magnet wire and the material of the coil spool. The heat generated in a coil must be transferred to the magnet core or to the coil surface and from there into the air. For a given ampere turns, a long coil of small diameter would promote heat transfer, but as the length of the coil is increased, the length of the magnet core must of necessity increase. As the magnet core increases in length, more and more magnetic flux linkages do not cross the airgap to the moving armature, and therefore do not contribute to the pulling force. If carried to extreme, such a long magnet core becomes a reactor or choke rather than a tractive magnet. Therefore, a more compact DC electromagnet having a lower watts loss while maintaining pulling forces equivalent to conventional DC electromagnets presently used would be beneficial.